The prove of alternate interior angles 3 and 6 are congruent is completed with vertical angle theorem, corresponding angles and transitive property of congruence.
How to complete the proof
The two column proof is written as follows
Statement Reason
∠ 3 ≅ ∠ 2 Vertical angles are congruent
∠ 6 ≅ ∠ 2 Corresponding angles are congruent
∠ 3 ≅ ∠ 6 Transitive property of congruence